Multiple σEcfG and NepR Proteins Are Involved in the General Stress Response in Methylobacterium extorquens.

Julia Frunzke,Anne Francez-Charlot,Andreas Kaczmarczyk,Julia A Vorholt,Judith Zingg,Willem Van Schaik

doi:10.1371/journal.pone.0152519

Abstract

In Alphaproteobacteria, the general stress response (GSR) is controlled by a conserved partner switch composed of the sigma factor σEcfG, its anti-sigma factor NepR and the anti-sigma factor antagonist PhyR. Many species possess paralogues of one or several components of the system, but their roles remain largely elusive. Among Alphaproteobacteria that have been genome-sequenced so far, the genus Methylobacterium possesses the largest number of σEcfG proteins. Here, we analyzed the six σEcfG paralogues of Methylobacterium extorquens AM1. We show that these sigma factors are not truly redundant, but instead exhibit major and minor contributions to stress resistance and GSR target gene expression. We identify distinct levels of regulation for the different sigma factors, as well as two NepR paralogues that interact with PhyR. Our results suggest that in M. extorquens AM1, ecfG and nepR paralogues have diverged in order to assume new roles that might allow integration of positive and negative feedback loops in the regulatory system. Comparison of the core elements of the GSR regulatory network in Methylobacterium species provides evidence for high plasticity and rapid evolution of the GSR core network in this genus.

Highlights

The general stress response (GSR) is a widely conserved response in bacteria for environmental adaptation
M. extorquens AM1 possesses a rather complex PhyR-NepR-σEcfG cascade compared to other Alphaproteobacteria
Our study identifies two σEcfG sigma factors, σEcfG1 and σEcfG2, as having a major contribution to the regulation of the GSR, whereas up to four of the remaining σEcfG proteins are involved in the response, but to a lesser extent

Summary

Introduction

The general stress response (GSR) is a widely conserved response in bacteria for environmental adaptation. It is defined as a preventive response induced by diverse stressful conditions and is generally governed by complex regulatory systems [1,2,3]. In Alphaproteobacteria, the GSR is controlled by a sigma factor of the ECF (extracytoplasmic function) family generally called σEcfG [4]. The three proteins function in a partner switch: in absence of stress, σEcfG is sequestered by NepR while PhyR is inactive. PhyR becomes phosphorylated and binds NepR, releasing σEcfG that can associate with the RNA polymerase core enzyme to transcribe stress-related genes (for review, [1, 5]).

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Conclusion